perf: add per-request RequestCache to is_user_allowed #822

Job	Run time
tests (ubuntu-latest, 3.12, quality)	59s
tests (ubuntu-latest, 3.12, django52)	1m 44s
tests (ubuntu-latest, 3.12, docs)	53s
tests (ubuntu-latest, 3.12, pii_check)	31s
	4m 7s

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